Centrifugal pumps have high capacity, measured in gallons per hour (gph), are readily available, and are generally inexpensive. They are, however, unsuitable for de-watering because de-watering requires intermittent to extended periods of dry operation. An example of a de-watering scenario is the draining of a thin mud slurry sloughed onto a recovery mat while washing trucks, or other vehicles.
Reciprocating diaphragm pumps can be operated dry and are suitable for dewatering. They have low capacity, however, and can be quite expensive. Compared to the least expensive diaphragm pump, a centrifugal pump could cost 70%-80% less while having twice the head and nearly four times the capacity for throughput.
The impeller of a centrifugal pump has a shaft which extends through a housing to an external motor. The housing, which is a chamber containing the volute, is rendered water-tight by means of a seal surrounding the shaft, or the bearing in which the shaft is situated. The seal can be degraded by the heat of friction, which is normally dissipated by the circulation of fluid through the chamber. The seal can be quickly destroyed, however, during a period of dry running, such as may occur when water is draining from a slurry into a pool which is periodically pumped dry.
U.S. Pat. No. 5,667,357 to Buse et al discloses a system for providing a flow of water to the bearing seal during intermittent periods of dry running. The water is recycled from a reservoir connected to the output port of the pump. The reservoir is effectively a bulge in an ascending pipe which retains a body of water therein. The pressure gradient, or head, caused by the rotating impeller, drives a recirculation current from the body of water through a pre-existing channel system, designed to externally irrigate the bearing casing, back to the volute chamber.
U.S. Pat. No. 4,773,823 to Pease teaches a similar system, except that the object in this case is to increase the life of the seal rather than to sustain dry running periods. The re-circulated water is drawn not from a reservoir, but directly from the volute. Since there is always a torus of water in the volute, even during dry running, conceptually there is a supply, at least for short periods, for continuous recirculation.
In both Buse and Pease, the cooling effect is achieved by water which has been previously heated and recycled. The system, therefore, is thermally inefficient and, in some sense, self-defeating. In addition, the porting through the bearing housing for channeling the recirculation would weaken the very structure which is under dynamic stress. What is needed is a simple method of providing a continuous flow of fresh water through the pump when the suction is otherwise dry-cycling.